Electron-impact emission spectroscopy (EIES) is a versatile technique for monitoring the vapor flux density in a vacuum environment by measuring characteristic photon emission from such species. The EIES technique may monitor multiple materials simultaneously using a single sensor which makes EIES useful for fabricating thin film products such as high-temperature superconductors, compound semiconductor devices, and solar cells, as examples. More specifically, EIES may be used for controlling film composition during co-deposition processes for the fabrication of multi-component thin film materials including the high-efficiency CIGS (copper-indium-gallium-diselenide) photovoltaic cells, as an example.
One or more outer shell electrons of vapor phase atomic or molecular species may be excited to higher energy levels when these species are subjected to bombardment by low-energy electrons. Such excited electrons emit photons having specific energies when returning to lower energy states which are characteristic of the atomic or molecular species. Further, the intensity of a particular emission line is proportional to the particle number density of the species generating this line. The measured intensity may therefore be converted to a deposition rate in the situation where the emitted photons are emitted from a flux of atoms or molecules being deposited on a substrate.
Emission spectra of most species consist of multiple lines and/or bands. The high specificity of EIES for atomic species derives from the unique emission spectrum of each element and the narrow widths of atomic emission lines. Well-spaced, non-overlapping emission lines may generally be allocated to different atomic species. For practical reasons, the preferred emission line is typically the one with the strongest intensity.
By contrast, the electron-impact emission spectra of many molecular species are often broadband in nature. When optical emission from an EIES sensor is measured using a wavelength selection device having a chosen resolution, such as an optical filter or monochromator, as an example, the emissions from common residual gases may interfere with those from the vapor flux and cause erroneous flux measurements. Such interference is most pronounced when measuring low-flux densities in the presence of gases such as are present in reactive deposition processes.